1. Field of the Invention
The present invention relates to an electromagnetic clutch that cuts and supplies power and that is particularly suitable for driving a compressor in a refrigeration cycle for an automobile air conditioning apparatus.
2. Description of Related Art
One type of known electromagnetic clutch has been generally utilized as a coil-fixed type clutch which has an electromagnetic coil on a fixed member for generating an electromagnetic suction force. This coil-fixed type electromagnetic clutch forms a passage for supplying current to the electromagnetic coil without an additional current supply member, because the electromagnetic coil is installed in the fixed member.
However, amagnetic circuit, through which flux passes, is formed through the fixed member, the rotational member and an armature magnetically attracted by the drive-side rotational member because the electromagnetic coil is installed in the fixed member. Thus, magnetic gaps are formed among these three circuit components. Therefore, the size of the electromagnetic coil may be increased, or it consumes more electricity, due to greater magnetic loss and lower magnetic efficiency.
To solve this problem, a coil-rotation type which includes the electromagnetic coil on the rotational member is proposed in, for example, JP-U-1-131028 and JP-U-2-54928. According to these coil-rotation type, the magnetic circuit comprises only the rotational member and the armature magnetically attracted by the rotational member. Therefore, such coil-rotation type has an advantage because the magnetic loss is much smaller and its magnetic efficiency is much higher than the coil-fixed type.
However, according to the electromagnetic clutch disclosed in JP-U-1-131028, a size in the axial direction of the electromagnetic clutch is large. Also, installation of the electromagnetic clutch in small installation areas such as an automotive engine room is difficult because the current supply members that supply current to the rotating electromagnetic coil, such as a slip ring and a brush, are located on an outer axial portion of the electromagnetic coil. Furthermore, the life of the brush is short because the brush is mounted at a location where the distance from the rotational center of the electromagnetic clutch is comparably large, and the peripheral velocity of the brush is large.
According to the electromagnetic clutch disclosed in JP-U-2-54928, a size in the axial direction of the electromagnetic clutch is also large because the current supply members, such as a slip ring and a brush, are located on an outer axial portion of a slave device drive shaft, such as a compressor.